"I tell my friends and family that I'm going to place X, Y, or Z, but I don't tell them my logic behind it," says one member of this intriguing club.

On a crisp Sunday night in late November, Clarence Ing, M.D., set off on a long-planned trip to Singapore. At California's Sacramento International Airport, instead of taking a flight across the Pacific Ocean, he boarded a Delta (NYSE:DAL) jet bound for Atlanta. There he caught a connecting flight to New York, then to Tokyo, and (finally), almost 35 hours after he first took off, he arrived in Singapore.

But Ing didn't grab a taxi and head to a hotel. Instead, he hung out at Singapore's Changi Airport for three hours before promptly heading home, tracing the same route back. He landed in Sacramento on Wednesday night after spending about three days solely on airplanes and in airports.

Ing, whose ticket was entirely paid for with points from his Visa (NYSE:V) credit card (which he thinks were worth sacrificing for the greater cause), gained 24,523 frequent-flyer miles for his whirlwind, round-the-globe trip, with which he was extremely satisfied. The 75-year-old had embarked on what's known in the airline industry as a "mileage run," or cheap flights taken solely to accumulate frequent-flyer miles, with no regard for the destination.

"I chose to fly to Singapore because that's how you can accumulate the most miles in the least amount of time," explains Ing, a preventative medicine practitioner in Sacramento. "It involves flying from the West Coast to the East Coast and then back across, so that's a lot of miles."

Passengers typically go on such runs to rack up enough frequent-flyer miles to qualify for "elite" status in an airline's frequent-flyer program, which often includes perks such as flight upgrades, airline lounge access, and priority boarding.

Ing, for example, has achieved top-tier Diamond Medallion status with Delta Air Lines. To remain a Diamond member this year, he needed to notch 125,000 miles with the airline by the end of 2013. Because he travels often to lecture at medical conferences, the bulk of his frequent-flyer miles are accumulated through "regular" flights. Ing goes on mileage runs to various destinations a couple of times a year to ensure he'll cross the 125,000-mile threshold for Diamond status. In 2013, he racked up about 156,000 miles, exceeding the airline's requirement."But that's OK, because [frequent-flyer miles] roll over on Delta. On American Airlines and United Airlines they don't, so once you get to where you want to be in terms of miles, you don't want to fly anymore."

What would be considered a value-for-money airfare for mileage runs? It depends on a metric called the cost per mile (CPM), which is the price of an air ticket divided by the number of frequent-flyer miles you would gain. According to mileage runner Athan Slotkin, a business development director at a loyalty management company, any airfare that has a CPM of $0.05 or lower is worth snapping up if you need the miles.

While some mileage runners like Ing prefer "pure" runs where they don't even venture out of their destinations' airports to cut down on travel time, others take a more relaxed approach: They snap up cheap plane tickets where they can find them, but they also spend a day or two enjoying their destinations before heading home.

Adam Kornfield, a 31-year-old New York-based online entrepreneur, is in the latter category. He says he always tries to find low-priced fares to places he has never been before, even if he might only spend half a day there.

"At the end of 2011, I had gold status on United and was almost platinum," he recalls. "So I asked my friend, who flies often, 'Dude, I'm almost platinum. Should I go for it?' He was like, 'Do it, do it!' So I booked a cheap ticket to go to Istanbul on Turkish Airlines [a United partner airline] over the three-day Christmas weekend and stayed for two nights. I met a bunch of people, had a great time, and got my platinum status."

Not all mileage runners get the same support from friends and family, mind you. "My family probably thinks I'm more lunatic than reasonable," says Slotkin, who began going on mileage runs six years ago when he was in graduate school. "But I like to fly, and I travel a good amount for work, so I'd like flying to be as comfortable and as smooth as possible. I'd [also] like to get the most value out of it. [Mileage running] is the way to do it. I think when my family takes a step back and understands the context, they know it makes sense."

Kornfield says he's more discreet about sharing his mileage-running adventures. "I tell my friends and family that I'm going to place X, Y, or Z, but I don't tell them my logic behind it," he says. "They ask, 'What are you up to this weekend?' and I just say, 'Oh I'm going to Vegas.' I don't really get into the details."

For Ing, mileage running gets the seal of approval from his wife, May, because she does it as well. A dietician who travels occasionally for work, May accompanies Ing on some of his shorter mileage runs to hit the 75,000-mile requirement for Delta Platinum status.

And what do their five adult children think? "At my stage in life, my kids are not going to tell me what to do," Ing quips.

A Strong Community

If mileage runners are misunderstood by friends and family, they can always turn to their fellow "runners" -- both online and in person -- for camaraderie. On frequent-flyer discussion forums like FlyerTalk.com and MilePoint.com, Ing and Slotkin post items about flight offers and discuss successful strategies. There are even conventions where people can come together to learn about the subject. One is the twice-yearly Frequent Flyer University event organized by Gary Leff, an airline industry expert and co-founder of MilePoint.com. "We held one in April in DC and over 600 people attended," he says.

Having served as moderator on FlyerTalk for many years, Leff has heard some crazy trips. "Five or six years ago, one person flew 100,000 miles between New York, Portland, and London in the first 17 days of January to earn his top-tier elite status in as few days as possible," Leff says. "He pretty much never left the airport."

Another impressive mileage run that Leff remembers was nicknamed Baht Run on the online forum. "You used to be able to buy domestic flights in Thailand for as little as $8," Leff says. "And Thai Airways is a partner of United. So there were people who were flying 10 flights a day for 10 days between Chiang Mai and Chiang Rai in northern Thailand to earn top-tier elite status on United for about $800. This was about 11 or 12 years ago."

What Do Airlines Think About Mileage Runs?

Given the strong interest in mileage runs -- FlyerTalk.com sees some 27,000 to 52,000 unique visitors a day -- there's no doubt that airlines are well aware of the practice. What do they think of it?

Leff says that airlines welcome mileage runners. "The thinking [for the airlines] is: This is extra revenue we would otherwise never get. And these are customers that are incredibly excited about our brand and our program," he explains. "I asked the head of the United frequent-flyer program about mileage runs years ago, and he smiled and said, 'Go for it.'"

Though mileage runners buy cheap tickets, Leff says they're not necessarily less-profitable customers compared with others who purchase higher-priced tickets. "If somebody is buying the last seat on a plane [for] a high price, that might have been a last seat that would've been purchased by someone else at a high price anyway," Leff says. "So there's not really an economic profit there. In fact, lower-revenue flyers are surprisingly profitable because they're taking up seats that would've otherwise gone empty. All the marginal revenue they contribute goes straight to an airline's overall profit."

Leff points out that airlines are also making mileage running more difficult. Both Delta and United have announced that starting next year, customers not only have to fly a minimum number of miles to earn elite status -- they must also spend a minimum number of dollars. To qualify for Diamond Medallion status with Delta, customers must have paid $12,500 in airfare on top of flying 125,000 miles. The new rule makes it more difficult for customers to rack up miles purely with cheap flights.

Still, there are ways to get around these new revenue requirements. If you have a Delta co-branded American Express card and you charge $25,000 to the card, Delta will waive the $12,500 spending requirement, says Leff.

Ultimately, Slotkin says that mileage runners will always manage to outwit airline frequent-flyer program developers and find cheap deals no matter what they try to do to prevent it. "It's as if you have a bunch of computer hackers out there and you have an antivirus program," he explains. "At the end of the day, companies are going to put out antivirus software, then hackers will figure out a way around it. Companies will then put out new software, and on and on it continues." However, he adds that comparing mileage runners to hackers isn't the best analogy: "That's not who we are."

Seven Tips to Become a Pro "Mileage Runner"

So you want to be a mileage runner and enjoy elite status on an airline while spending the least amount of money possible? Here are some tips that experts say you should follow:

Make sure you enjoy the experience of flying on some level. "If you don't enjoy it at all, you shouldn't do it," says Slotkin.

Figure out why you want to do it. How much are the benefits of elite status worth to you? Compare them to the cost of tickets and the cost of your time to figure out if the economics work for you.

Plan early in the year. Get an idea of how often you'll fly for official work or pleasure, calculate how many miles you still need after those flights to hit a certain status, and try to fill the gaps with mileage runs.

Monitor websites for good deals. Leff recommends a blog called TheFlightDeal.com. "It does as good a job as any I've seen in coming up with the cheapest flights every day from every city," he says.

Read and participate in forums like FlyerTalk.com to learn from others. "There are a lot of people who've done it for a long time, so it would make your life a lot easier reading stuff on forums," Slotkin advises.

If you find a good deal, don't hesitate: Book it. If you have elite status, you can usually cancel the ticket within a day or two and still get a full refund if you change your mind. This is especially crucial, Slotkin says, for mistake fares that airlines sometimes publish online. Customers who book them the earliest are much more likely to get mistake fares honored.

On mileage runs, make sure you have plenty of entertainment and/or plenty of work to do so you can be be productive. The most important tip? "Have a sense of adventure to meet people along the way," says Leff.

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